cmd/compile: use typed goret for stack ABI results

[zhouguangyuan0718]

Summary

• use the Go ABI allocator as the source of truth for results: register-assigned results remain direct LLVM returns, while each non-empty stack-assigned result becomes a typed, aligned goret pointer parameter with "goretindex"="N"
• allocate caller-owned homes for memory results and use the same home for ordinary SelectN, SelectNAddr, direct calls, indirect/closure calls, and tail-return adaptation
• bind non-escaping named stack results directly to their goret home, eliminating the callee alloca and final self-copy
• retain Go escape semantics for heap-escaped named results: mutate one heap object inside the callee, then copy its final value into the goret home at return
• preserve goret through Go statepoint validation and treat the fixed result home like existing alloca/byval fixed objects during relocation repair
• rewrite statepoints after standard codegen IR preparation and immediately before SelectionDAG, then split every statepoint continuation
• rematerialize direct alloca-derived memory addresses in their continuation blocks so SelectionDAG cannot CSE a stale stack address across a growing call
• update ABI0, cgo unsafe-args, linkname wrappers, deferred named results, GoObj/stack-map fixtures, and statepoint CFG expectations for the new contracts

This change intentionally establishes the result ABI first. Caller result homes are still ordinary entry allocas; forwarding or otherwise eliding those homes is a separate optimization and is not required for correctness.

Validation

• native Darwin/arm64 standard Go toolchain build against the coordinated static LLVM development payload
• cmd/compile/internal/ssa: pass
• statepoint plugin CTest, including focused IR/MIR continuation and direct alloca-memory checks: 97/97 pass
• ABI differential test: pass through raw IR, O2/statepoint rewrite, MIR, GoObj, pointer maps, and PCSP
• changed LLVM codegen FileCheck fixtures: 4/4 pass
• full ABI fixture built with LLVM O2, linked, and executed successfully on native arm64
• heap-escaped named memory result regression verifies both in-function aliasing and independence of the returned value after the escaped heap object is mutated post-return
• archive/tar passes with -gcflags=all=-enablellvm\ -llvmironly and default<O2>; TestReadTruncation also passes for 10 consecutive runs

Joint linux-amd64/linux-arm64 CI selects the immutable payload produced from the exact LLVM PR head below.

LLVM-PR: goallc/llvm-project#88

Additional standard-library qualification after LLVM GoObj relocated-private-constant support:

• Darwin/arm64 O2 dependency-closure tests pass for compress/flate, compress/gzip, compress/zlib, and archive/zip
• the four packages move from the common blacklist to the common whitelist
• stdlib policy uses one -gcflags=all=-enablellvm\ -llvmironly flag per test invocation; dependency enumeration and per-package flag expansion were removed after direct all-LLVM flate and runtime entry tests passed